1. Field of the Invention
The invention relates to a determination system and determination method for determining whether metal lithium is precipitated in a lithium ion secondary battery, and a vehicle equipped with the determination system.
2. Description of the Related Art
Generally, the internal state of a lithium ion secondary battery is inspected. As one internal state of the lithium ion secondary battery, it is intended to recognize through inspection whether metal lithium is precipitated. For example, a phenomenon called “dendrite precipitation” in Japanese Patent Application Publication No. 8-190934 (JP-A-8-190934) is also the precipitation of metal lithium. A lithium ion secondary battery originally does not contain metal lithium; however, depending on usage, metal lithium may precipitate on a surface of a negative electrode. A lithium ion secondary battery that has reached such a state needs to be replaced because deterioration in performance is remarkable. Therefore, it is necessary to recognize whether metal lithium is precipitated in a lithium ion secondary battery.
A method of detecting the internal state of a lithium ion secondary battery is, for example, described in Japanese Patent Application Publication No. 2003-59544 (JP-A-2003-59544). In the method described in JP-A-2003-59544, the lithium ion secondary battery first undergoes constant current charge and subsequently undergoes constant voltage charge. Then, a difference in index, such as charged capacity and internal resistance, between the charged battery and a standard battery is obtained. The internal state of the inspected battery is intended to be recognized on the basis of the above difference.
However, in order to apply the above related art, a power supply for charging an inspected battery needs to be compatible with both constant current charge and constant voltage charge. For this reason, a complex power supply system is required. Particularly, for example, in the case of in-car application, it is difficult to perform required charge by an electrical system of a vehicle itself. Generally, the electrical system of a vehicle is not designed for constant voltage control.
In addition, information about a decrease in capacity or an increase in internal resistance may be acquired as a result of inspection; however, it is impossible to determine what factor gives the acquired information. That is, not only in the case of precipitation of metal lithium but also in the case of normal usage degradation (hereinafter, referred to as “cycle degradation”) that is not attended with precipitation of metal lithium, the tendency of a decrease in capacity or an increase in internal resistance is observed. Therefore, it has been difficult to determine whether metal lithium is precipitated.